A lighting circuit of a discharge lamp, such as a metal halide lamp used as an automotive lighting source, includes a DC voltage increasing circuit having a DC-DC converter, a DC-AC conversion circuit (a so-called inverter), and a starting circuit. (See, e.g., Japanese patent document JP-A-7-142182.)
During lighting control of a discharge lamp, an unloaded output voltage (hereinafter referred to as “OCV”.) is controlled before the discharge lamp is lit. The discharge lamp is lit by applying a starting signal through the use of a starting circuit. Thereafter, the lamp is shifted to a steady lighting situation by reducing transient electric power applied to the discharge lamp.
In the DC voltage boosting circuit, for example, a switching regulator with a transformer is used. In addition, a full bridge type configuration using multiple pairs of switching elements, is mentioned for use as the DC-AC conversion circuit.
In a configuration mode of carrying out 2-stage conversions (i.e., DC voltage conversion and DC-AC conversion), the circuit size becomes large, and is unsuitable for small size circuits or devices. As a result, other configurations have been suggested in which an output is supplied to a discharge lamp with the voltage boosted by 1-stage voltage conversion in a DC-AC conversion circuit.
For example, in an arrangement equipped with a series resonance circuit using a capacitor and an inductance element, it is possible to control the electric power applied to the discharge lamp by changing the operating frequency of a half-bridge (i.e., drive frequency of a switching element), which forms a DC-AC conversion circuit, based on the fact that impedance of the circuit changes depending on frequency.
Assuming that inductance, which is related to a series resonance circuit, is described as “L” and the electric capacitance of a resonance capacitor is described as “C,” the resonance frequency “f0” is represented by “f0=1/(2.π.√(L.C)),” and has a nearly symmetrical frequency characteristic with a central focus on f0. To obtain stable circuit operation, it is preferable to carry out electric power control by changing the drive frequency of a semiconductor switching element which forms the DC-AC conversion circuit in a frequency range higher than f0.
In a frequency range higher than the resonance frequency f0 (inductive domain or delayed phase domain), there is a tendency that, as applied electric power increases, there is a decrease of frequency. Therefore, it is possible to form a feedback control system by obtaining applied electric power (targeted through calculation), and changing the drive frequency of a switching element on the basis of variation of its result and actual output electric power.
To increase electric power applied to a discharge lamp when carrying out the foregoing feedback control in a higher frequency range than the resonance frequency at the time of turning on the discharge lamp, it is acceptable if the drive frequency is decreased. However, if the frequency becomes less than the resonance frequency, then when drive frequency is decreased, applied electric power falls off. In summary, in a frequency range lower than the resonance frequency f0 (capacitive domain or advanced phase domain), there is a tendency that applied electric power decreases with decrease of frequency and, therefore, when it is kept unchanged, fading-away occurs due to a decrease of applied electric power.
Circuit design of an electric power system including a DC-AC conversion circuit, a resonance circuit, a transformer is carried out so that sufficient electric power can be applied to a discharge lamp, in a frequency range at the resonance frequency or higher In the past, it has been difficult to define the drive frequency in the following situations.                Situation where a power supply voltage to a lighting circuit decreases as a result, for example, of variation per hour or a change of surrounding environment, and it is not possible to output electric power at the targeted amount.        Situation there it is desired to carry out electric power supply under closed loop control to apply electric power to a discharge lamp by maximum capacity of a lighting circuit for facilitating growth of a discharge lamp arc, immediately after a starting high voltage signal is applied to a discharge lamp and the discharge lamp is activated.        
As the resonance frequency f0 is determined in dependence on “L.C” as described above, if the values of L and C are fixed, the value of f0 is also fixed and, therefore, it is acceptable if electric power control is not carried out in a frequency range less than f0, by placing a lower limit frequency so that the drive frequency does not become less than this value.
Resonance frequency is different with respect to each circuit, due to fluctuation of components which are used for the lighting circuit, and L value and C value change depending on the surrounding environment. Therefore, the value of the resonance frequency fluctuates.
To establish a minimum drive frequency for the lighting circuit in advance, it is possible to enlarge the margin error during design, or to adjust each circuit. However, in the former case, the circuit specification becomes excessive and cost increase. In addition, in the latter case, there is need to establish a lower limit frequency individually in mass production, which is not realistic.
The present invention addresses the situation where drive frequency becomes less than its minimum value, by automatically carrying out lower limit restriction of the drive frequency of a switching element, depending on a change of resonance frequency at the time of lighting-up, in a high frequency lighting circuit of a discharge lamp.